Microsoft has collaborated closely with US intelligence services to allow users' communications to be intercepted, including helping the National Security Agency to circumvent the company's own encryption, according to top-secret documents obtained by the Guardian.

The files provided by Edward Snowden illustrate the scale of co-operation between Silicon Valley and the intelligence agencies over the last three years. They also shed new light on the workings of the top-secret Prism program, which was disclosed by the Guardian and the Washington Post last month.